System pressure goes from high to low when heating off (E119)

[staithes14]

We have a Baxi condensing boiler which works OK, or did. Recently, there have been several instances where the system hasn't come on (no heat and no hot water to taps) because the boiler pressure has dropped, giving an E119 error code. Each time, I have let a bit of water in using the top-up loop, and the boiler has kicked in straight away. It works OK but then the pattern repeats itself.
Yesterday morning I did this and then when I came home from work it was showing E119 again, less than 11 hours after topping up the pressure! Then I had the same thing this morning, so the problem seems to have got worse.
One thing I noticed is that when I open the taps on the top-up loop, the pressure goes up very quickly - more so than used to be the case; a couple of seconds and it's right at the top of the green zone.
The central heating was not used from April through to September, but it worked OK until just before xmas, when the first E119 occurred.
A friend suggested there's probably a pressure reservoir somewhere in the boiler which can be "topped up with a bike pump" to use his phrase. Not sure how you'd do that but as it's a water pressure thing, I feel it's probably something I can do myself, being pretty handy.

 

[Gasguru]

It's indicative of a flat expansion vessel. You'll probably find the safety valve is leaking too and will need attention...check the small pipe to the outside wall. A drip a minute will drop the pressure.
Time to call someone in.